1. Field of the Invention
The present invention relates generally to the technology of measuring the non-methane hydrocarbon concentration in the emissions of an automotive internal combustion engine, and more particularly to the use of metal oxide semiconductor catalysts sensors to monitor the non-methane hydrocarbon oxidation efficiency of an exhaust system's catalytic converter.
2. Description of the Related Art
Catalytic converters have been used on gasoline-fueled automobiles produced in the United States since the mid-1970's for the purpose of promoting the oxidation of unburned hydrocarbons (HCs) and of carbon monoxide (CO). Soon after their introduction, the converters were adapted to promote the chemical reduction of oxides of nitrogen (NO.sub.x). At the present time these converters typically employ small amounts of platinum, palladium and rhodium dispersed over a high surface area particulate carrier vehicle which, in turn, is distributed as a thin, porous coating (sometimes called a washcoat) on the wall of a ceramic monolith substrate. The substrate is typically formed by an extrusion process providing hundreds of thin wall, longitudinal parallel open cells per square inch of cross section. These flow-through catalytic devices are housed in a suitable stainless steel container and placed in the exhaust stream under the vehicle downstream from the engine's exhaust manifold.
Under warm, steady-state engine conditions, this conventional catalytic converter containing the precious metal based three-way catalyst (TWC), so called because it simultaneously affects the oxidation of CO and unburned HCs and the reduction of NO.sub.x, effectively and efficiently removes most of tie automotive emissions. However, the catalyst system may become malfunctioning after experiencing thermal aging at an unusually high temperature, high exposure to poisoning gases like SO.sub.2, and Pb, etc. Furthermore, new emissions regulations require an extended durability of the catalytic converter from 50,000 miles to 100,000 miles. Lastly, as a means to ensure that vehicles meet the certified emission standards throughout the vehicle's operation life, On-Board Diagnostics-II (OBD-II) regulation, as passed by the California Air Resource Board (CARB), calls for continuous monitoring of the efficiency of catalytic converters by direct measurement of the hydrocarbon emission in the exhaust system after the catalyst light-off. Specifically, the monitoring system should be able to indicate when the catalyst system is malfunctioning and its conversion capability has decreased to the point where either of the following occurs: (1) HC emissions exceed the applicable emission threshold of 1.5 times the applicable Federal Test Procedure (FTP) HC standard for the vehicle; and (2) the average FTP Non-methane Hydrocarbon (NMHC) conversion efficiency of the monitored portion of the catalyst system falls below 50 percent.
On the other hand, automotive emissions, before the catalyst system has warmed up to operational temperatures, namely, cold start emissions contribute the majority of pollution from automobiles. Approaches such as, catalytic converters, close coupled to the engine, which heat and begin to function within a few seconds, electrically heated catalytic converters and in-line adsorbers which temporarily store unburned hydrocarbons until the catalytic converter lights off, have all been proven to be effective solutions for the reduction of cold start emissions. Again, OBD-II regulations require that systems be installed in the exhaust system to directly more or the functional status of any of these "cold-start" devices during the lifetime of the car (100,000 miles).
The use of hydrocarbon sensors as on-board catalytic efficiency monitors is a relatively new technological area which has generated increasing interest for the auto industry as a result of OBD-II legislation.
U.S. Pat. Nos. 5,408,215 (Hamburg et al.), 5,265,417 (Visser et al.) 5,444,974 (Beck et al.) each disclose a hydrocarbon sensor system, however each of these three systems utilizes a non-selective or "total" calorimetric catalytic sensor which not only oxidizes the HC but also oxidizes CO and H.sub.2. Given the fact that a properly functioning catalytic converter, after light-off, typically produces an exhaust gas hydrocarbon concentration, which is typically on the order of tens (or below) ppm, none of these diagnostic systems are capable of directly and selectively measuring HC concentration in this concentration range. Specifically, these sensor systems do not compensate or account for these interfering gases, especially CO, which are present in concentrations far greater than the HC and therefore interfere with the ability to accurately measure the HC concentration.